Apparatus and method for reducing thermal interference in MR heads in disk drives

ABSTRACT

An apparatus (and method) is provided that reduces thermal interference in the read signal of a disk drive. A variable or programmable resistance is used to change the transfer function of a filter in the read channel of the disk drive to filter the read signal. The filter has a first transfer function (first cut-off frequency) related to the programmed resistance during normal operation of the disk drive (i.e. when thermal interference is not detected). When thermal interference is detected in the read signal, the resistance is programmed to another value resulting in the filter having a second transfer function (second cut-off frequency). The resistance element is variable or programmable to different values resulting in different programmable transfer functions (or one of a multitude of cut-off frequencies) for the filter. In the preferred embodiment, detection of thermal interference increases the cut-off frequency of the filter thereby filtering, or reducing the effects of, the thermal interference in the read signal.

TECHNICAL FIELD

The present invention relates to disk drives and, in particular, to anapparatus and method for reducing thermal interference (thermalasperity) in magneto-resistive heads in disk drives.

BACKGROUND

Magneto-resistive and giant (G) magneto-resistive (collectively, MR)heads used in hard disk drive applications utilize a resistive sensingelement to read magneticly stored data from a storage disk. The MR headsare thermally sensitive and the resistance of a MR head changes withtemperature. The resistance of a typical MR head is given by theequation R=R₀(1+β(T-298)), where R₀ equals about 50 ohms, β is thethermal coefficient of the head, and T is temperature in Kelvin.Accordingly, any variations in temperature increase/decrease theresistance of the MR head and affect the readback signal generated bythe MR head as it reads data from the disk.

During operation of a disk drive, thermal interference occurs. A thermalinterference event is also referred to as a thermal asperity. Foradditional description and discussion of thermal interference or thermalasperities, reference is made to a published article entitled“Electronic Abatement of Thermal Interference in (G)MR Head OutputSignals”, IEEE Transactions on Magnetics, Vol. 33, No. 5, September 1997by Klaassen, et al., and is incorporated herein by reference. Onesolution proposed therein is to increase the low-frequency cutoff of ahigh pass filter, by switching in a resistor to lower the effectiveresistance of the RC high-pass filter—thereby increasing the cutofffrequency, when a thermal asperity is detected (the high pass filter istypically used to filter out low frequency signals in the read signalfrom the MR head). A significant problem with this solution is caused bythe switching out of the resistor after the thermal asperity event hassubsided to a certain level. This creates ringing and an unwanted offsetin the read signal which may cause the loss of data. In addition, thereis no control over the value of the resistor (or accuracy of theresistance value) that is switched into the circuit to lower theeffective resistance of the high pass filter.

Accordingly, there exists a need for a thermal asperity recovery circuitand method that eliminates or reduces ringing and any unwanted offset inthe read signal of a disk drive. In addition, there is a need for anapparatus and method that uses a programmable or variable cutofffrequency in a high pass filter used in thermal asperity recovery.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a thermal eventrecovery circuit for filtering a read signal from a read head in a diskdrive. The circuit includes a detection circuit for detecting a thermalevent that affects the read signal and a filter. The filter includes avariable resistance circuit having a resistance that varies in responseto a control input. The resistance is equal to a first predeterminedresistance in response to a first control input and equal to a secondpredetermined resistance in response to a second control input, and thesecond control input is generated in response to the detection of thethermal event. In another embodiment, there is provided a disk drivehaving a storage medium, a read head for detecting data on the storagemedium and generating a read signal, and the thermal event recoverycircuit described above.

In another embodiment of the present invention, there is provided athermal event recovery circuit for filtering a read signal from a readhead in a disk drive. The circuit includes a detection circuit fordetecting a thermal event that affects the read signal and a filter. Thefilter has a first predetermined transfer function in response to afirst input and a second predetermined transfer function in response toa second input, and the second input is generated in response to thedetection of the thermal event.

In yet another embodiment of the present invention, there is provided acircuit for detecting thermal interference and filtering a read signalfrom a read head in a disk drive. The circuit includes a detectioncircuit for detecting thermal interference affecting the read signal, acurrent source for outputting a first current or second current, and afilter. The filter includes a fixed resistive element for generating afirst resistance and a variable resistance circuit for generating asecond resistance. The second resistance varies in response to theoutput of the current source, and the second resistance is equal to afirst predetermined resistance in response to the first current and isequal to a second predetermined resistance in response to the secondcurrent. The second current is generated in response to the detection ofthe thermal interference.

In accordance with the present invention, there is also provided amethod of reducing the affects of a thermal asperity on a read signal ina disk drive. A read signal is filtered with a filter having a firsttransfer function. When a thermal interference is detected in the readsignal, the first transfer function of the filter is changed to a secondtransfer function. The read signal is filtered with the filter havingthe second transfer function. After a predetermined time period haselapsed after the detection of the thermal interference, the secondtransfer function of the filter is transitioned to the first transferfunction.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is made to the following description takenin conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a computer system and a disk driveutilizing the present invention;

FIGS. 2A-2B are graphs illustrating an output signal from an MR head andthe resistance value of the MR head before, during, and after a thermalasperity (heating event) occurs;

FIG. 3 is a block diagram of the read/write head and the pre-amplifiercircuit shown in FIG. 1 including a filter (and additional elements) ofthe present invention;

FIG. 4 illustrates a read signal with a thermal asperity andcorresponding TA fault signal and cut-off frequency change for thefilter;

FIG. 5 is a more detailed diagram of the filter of the presentinvention;

FIG. 6 is a graphical representation of the desired effective resistanceR_(eff) of the filter of the present invention;

FIG. 7 is a detailed schematic diagram of the programmable resistiveelement or circuit shown in FIG. 5; and

FIG. 8 is a schematic and block diagram illustrating the control circuitshown in FIG. 3 in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, like reference characters designate likeor similar elements throughout the drawings.

Now referring to FIG. 1, there is shown a block diagram of a disk drivesystem 10 including a disk drive 11 in accordance with the presentinvention. The disk drive 11 includes a storage medium in the form ofone or more disks 12, each of which may contain data on both sides ofthe disk 12. Data is stored on the disks 12 and/or read from the disks12 by one or more read/write heads 14. The read/write head 14 is an MRhead. The read/write head 14 is connected to an arm 16, with bothpositionally controlled by a Voice-Coil Motor (“VCM”) 18 and a positionsystem 20. The position system 20, through the VCM 18, positionallymaintains/moves the head 14 radially over the desired data on the disks12. As a signal (or signals) is input to the VCM 18, the VCM 18 movesthe arm 16, thereby moving the read/write head 14).

A read channel 22, including a pre-amplifier circuit 23, detects data inanalog signal form from the head 14 (the head 14 detects generates aread signal from the information stored on the disk 12) and converts thedata into digital form. A controller 24 recognizes and organizes thedigital data from the read channel 22 into bytes of data. An interfaceadapter 26 provides an interface between the controller 24 to a systembus 28 specific to the system used. Typical system buses include ISA,PCI, S-Bus, Nu-Bus, etc. The host system will also typically have otherdevices, such as a random access memory (“RAM”) 30 and a centralprocessing unit (“CPU”) 32 attached to the bus 28. A spindle motor(“SPM”) 34 and a SPM control circuit 36 rotates the disk 12 andmaintains the disk 12 at the proper speed.

Thermal events occurring during the reading of data from the disk 12cause large perturbations in the read signal resulting in the loss ofdata. When the perturbations are relatively short in duration,utilization of error correction coding (ECC) techniques allow for therecovery of the lost data. However, when these events last for asignificant period of time, ECC cannot recover the lost data. Sometimes,a re-read of the data sector can recover the data resulting in anincrease in the read time. Other times, the thermal event occurs due tothe disk surface at that location, which cannot be corrected by are-read.

Now referring to FIGS. 2A and 2B, there are shown two graphsillustrating the output signal from an MR head and the resistance valueof the MR head before, during, and after a thermal asperity (heatingevent) occurs. As shown, the MR head is operating under normalconditions until a time T1 when a thermal asperity (heating event)occurs. At the time T1, the resistance of the MR read head increases, asshown. The read signal increases similarly, as shown. As the thermalasperity slowly decays and the temperature of the MR head returns tonormal (or ambient), the resistance and the read signal change also, asshown in FIGS. 2A and 2B. A time period t shown in FIG. 2A illustratesthe decay time period for the thermal asperity, after which the data ofthe read signal is once again accurately read by the read channel.

The time for decay of the read signal back to normal after a thermalasperity occurs depends on several factors, including the size ormagnitude of the event causing the thermal asperity, the magnitude ofthe temperature increase, the ambient temperature, cooling effects ofthe surrounding environment, etc. As will be appreciated, thermal eventsoccurring in disk drives may be of any length and/or magnitude. Further,the thermal event may also be a cooling event, where the resistance ofthe MR head decreases.

It is not common for the decay of the read signal back to normal to takeup to about one microsecond (and even larger, up to 2-3 microseconds).During this time period, data on the read signal may be lost. At typicaldata bit rates for current hard drives of about 200 Mbit/sec, about 200bits or about 25 bytes of data (burst) may be lost due to a thermalasperity. Under typical industry standards, error correction may allowfor recovery of a lost data burst of up to about 5 bytes in length,depending on the ECC techniques used. Thermal asperities are a majorproblem at present, and increases in data read rates further exacerbatesthe problem.

Now referring to FIG. 3, there is shown a block diagram the read/writehead 14 and the pre-amplifier circuit 23 in accordance with the presentinvention. The read signal from the head 14 is input to an amplifier 100of the pre-amplifier circuit 23. A thermal asperity recovery circuit 102receives the read signal and outputs the read signal to a differentialamplifier 104. The thermal asperity recovery circuit 102 includes afilter 112 and a control circuit 110 for controlling the transfercharacteristics (or transfer function) of the filter 112. The output ofthe differential amplifier 104 is input to the read channel circuitry(not shown) and input to a thermal asperity detector 106 having adifferential comparator 108. When a thermal asperity is detected, asignal is generated by the detector 106 and output to the controlcircuit 110 for controlling the filter 114.

In general operation, the thermal asperity detector 106 monitors theread signal coming from the head 14. When there is a variation of theread signal voltage above (or below, when a cooling event occurs) apredetermined threshold, the differential comparator 108 is triggeredindicating a thermal asperity is detected (sometimes called a TA fault).As will be appreciated, the read signal may be a single-ended signal,and the invention described herein can be modified by those skilled inthe art to operate with a single-ended signal, however, differentialsignals are preferred. The output of the comparator 108 is not latched,allowing a TA length monitoring circuit (not shown) to monitor thelength of the thermal asperity. The predetermined threshold voltage ofthe comparator 108 is preferably generated by a 6-bit digital-to-analogconverter (DAC)(not shown) and data registers 122, including a 6-bitdata register for generating the 6-bit input to the DAC. In thepreferred embodiment, the 6-bit DAC provides an output voltage generallyin the range of 0 to 0.8 volts. The data registers 122 are programmedthrough an interface 120 (serial or parallel).

The programmability of the comparator threshold provides flexibility inselecting a threshold below (or above) which a thermal asperity having agiven magnitude will not generally cause data loss (i.e., those that donot pose a problem and can be corrected by ECC, thus no detection isnecessary). Additional data register bits may be used to enable/disablethe detector 106 and for TA fault reporting.

The comparator 106 includes a hysteresis function, as illustrated inFIG. 4. As shown at T1, a thermal asperity (heating event shown) occursand the read signal increases in amplitude and slowly returns to normalas the temperature of the MR head returns to normal (or ambient), asshown in FIG. 4. When the amplitude of the read signal rises above theTA threshold (the threshold of the comparator 108), a TA fault isdetected and the control circuit 110 activates the thermal asperityrecovery circuit 102 and the cutoff frequency (or transfer function) ofthe filter 114 therein (shown in FIG. 3) is increased from an initialcutoff frequency to a predetermined higher cutoff frequency, illustratedconceptually in FIG. 4.

As the thermal asperity slowly subsides, at a time T2, the amplitude ofthe read signal falls below a hysteresis threshold of the comparator106. At this time, the TA fault is no longer detected and the thermalasperity recovery circuit 102 is deactivated Deactivation results in thecutoff frequency of the filter transitioning to its initial cutofffrequency, identified at point T3This transition (from T2 to T3) maytake any form, however, including any one of the three following forms:(1) almost instantly (depending only on the physical limitations of thecircuits utilized), (2) linearly (first order, i.e., substantiallyconstant slope) over a period of time, or (3) nonlinearly (i.e., secondorder or higher, substantially non-constant slope, similar to thecharging/discharging curve of a capacitor) over a period of time.

If the transition from the higher cutoff frequency to the initial cutofffrequency occurs abruptly, similar to the abrupt transition from theinitial cutoff frequency to the higher cutoff frequency, an offset isgenerated in the read signal which is generally unwanted and may causeread channel data detection problems. However, in a preferableembodiment of the present invention, the transition (linearly ornonlinearly) from the higher cutoff frequency to the initial cutofffrequency occurs relatively slowly (over a period of time), unlike theabrupt transition from the initial cutoff frequency to the higher cutofffrequency. This reduces or eliminates the offset problem. Mostpreferred, the transition is nonlinear. The time period for thistransition can range anywhere from about 50 nanoseconds to about 1microsecond, and even longer, and preferably is between about 200nanoseconds and 700 nanoseconds. The circuitry and method ofgenerating/performing the transition is described below.

Now referring to FIG. 5, there is illustrated in more detail the filter112 of the thermal asperity recovery circuit 102, The filter 112includes a capacitor C1, a capacitor C2, and a programmable resistiveelement 130 including a resistor R1, a resistor R2 and a variableresistance circuit 132. The capacitors C1 and C2 and the resistiveelement 130 provide an RC-type high pass filter for the differentialread signal output from the amplifier 100. As will be appreciated, asingle-ended read signal could be optionally used, and the RC-typefilter would be a single-ended filter. However, differential signals arepreferred. It will be understood that filters other than an RC-typefilter may be used for the filter 112 (including second order and higherfunction filters), as long as the cut-off frequency varies with a changein resistance, but the preferred filter 112 is an RC-type high passfilter.

The resistors R1 and R2 are substantially fixed resistance resistors,and may include other devices that provide a resistance, such astransistors. In the preferred embodiment, the resistors R1 and R2 have avalue of 200 Kohms and are provided by MOSFETs (not shown). A biascircuit 134 provides a voltage reference or voltage bias to the filter112 (thereby biasing the MOSFETs used for the resistors R1 and R2), asshown in FIG. 5.

The variable resistance circuit 132 is coupled in parallel with theresistors R1 and R2 and provides a variable resistance. The variableresistance is programmable based on inputs from the control circuit 110.Characteristics of the RC filter are better understood with reference toa basic RC filter, therefore, the description shall utilize thecapacitor C1, the resistor R1 and the programmable resistance obtainedfrom the variable resistance circuit 132.

The resistor R1 and the programmed resistance of the variable resistancecircuit 132 provides an effective resistance R_(eff). The value of thecapacitor C1 and the effective resistance R_(eff) determines the cut-offfrequency of the high pass RC filter. In the present invention, resistorR1 and the capacitor C1 are substantially fixed, therefore, as theresistance of the variable resistance circuit 132 varies, so does thecut-off frequency of the filter 112.

Now referring to FIG. 6, there is shown a graphical representation ofthe desired effective resistance R_(cff) of the filter 112 of thethermal asperity recovery circuit 102. During normal operation of theread channel (i.e., no detected thermal interference), the effectiveresistance R_(eff) is relatively constant at R_(EFF), as shown. In thepreferred embodiment, during this period of time, the resistance of thevariable resistance circuit 132 is a relatively high impedance (coupledin parallel with the resistor R1), resulting in the effective resistanceR_(eff) of the filter 112 substantially equaling the resistance of theresistor R1 (R_(EFF)).

When a thermal asperity is detected at the time T1, the effectiveresistance R_(eff) of the filter 112 is lowered, thus raising thecut-off frequency of the filter 112. The effective resistance R_(eff) islowered by programming the variable resistance circuit 132 to apredetermined resistance in response to an input signal from the controlcircuit 110. For example, if the variable resistance circuit 132 isprogrammed to a value of 200 Kohms, the effective resistance R_(eff)will be 100 Kohms (R1 in parallel with variable resistance=200K inparallel with 200K=100K).

In the preferred embodiment, the variable resistance circuit 132 hasfour programmable resistance settings, with each setting resulting inone of four (R_(EFF), RA, RB, RC) different effective resistancesR_(eff) for the filter 112, and thus a different cut-off frequency.However, any number of programmed resistances may be used. As will beappreciated, the programmed resistances of the variable resistancecircuit 132 are selected to achieve the cut-off frequency(s) desired forthe filter 112. The programmable thermal asperity recovery circuit 132of the present invention provides flexibility thereby allowingutilization in read channels having potentially different or varyingcharacteristics (e.g., different data rates, different causes for thethermal interference, etc.).

At the time T2, and as described earlier, the TA fault is no longerdetected and the thermal asperity recovery circuit 102 is deactivated.Upon deactivation, the cutoff frequency of the filter transitions to itsinitial cutoff frequency, identified at point T3. In the preferredembodiment, the deactivation is accomplished by switching in the highimpedance (or open circuit) in parallel with the resistor R1, that ispresent in normal operation (no thermal asperity detected).

Now referring to FIG. 7, there is illustrated a detailed schematicdiagram of the programmable resistive element or circuit 130 includingthe previously described resistors R1 and R2 and the variable resistancecircuit 132. The resistance of the resistive element 130 is programmablein response to the magnitude of an input signal from the control circuit110. In the preferred embodiment, a current I_(s) controls theresistance of the variable resistance circuit 132, which in turn variesthe resistance of the resistive element 130 of the filter 112.

The variable resistance circuit 132 includes a combination oftransistors 200, 202, 204, 206, 208, 210 and resistors (six 2K ohmresistors) interconnected, as shown in FIG. 7, in a configurationresulting in a resistive element coupled in parallel with the fixedresistors R1 and R2. The resistance of the variable resistance circuit132 depends on the magnitude of the current flowing through thetransistors 200, 202, 204, 206, 208, 210. The transistors 200, 202, 204,206, 208, 210 each act as a resistive element when current flows throughthem, with the resistance value depending on the magnitude of thecurrent flow therethrough and device characteristics. When the currentI_(s) is essentially zero, the resistance of the transistors is high(high impedance) resulting in a resistance of the filter 112substantially equal to the value of fixed resistors R1 and R2. In thepreferred embodiment, the transistors 200, 202, 204, 206, 208, 210 arebipolar transistors. It will be understood by those skilled in the artthat the characteristics of the transistors should be selected based onseveral factors, including the desired resistance(s) of the variableresistance circuit 132 (and ultimately the resistance R for the highpass filter 112) and the magnitude of the current Is that is utilized.

Now referring to FIG. 8, there is illustrated one embodiment of thecontrol circuit 110 for programming/setting the resistance of thevariable resistance circuit 132 (and ultimately programming/setting thecut-off frequency/transfer function of the filter 112). The controlcircuit 110 includes a first current source (A) 220 and a second currentsource (B) 222. The current sources 220 and 222 provide sources ofcurrent (or reference current), and output a substantially fixed firstcurrent I_(A) and a substantially fixed second current I_(B),respectively. These currents are summed together to provide the currentI_(s) input to the resistive element 130 (see FIG. 7). Accordingly, thecurrent source A 220 and the current source B 222 provide a programmablecurrent source outputting the programmable current I_(s). In thepreferred embodiment, the current I_(s) equals zero, I_(A), I_(B), orI_(A)+I_(B). As will be appreciated, additional current sources, such asC, D, E, etc. could be used to increase the programmability.

Operation of the control circuit 110 is enabled/disabled by operation ofthe transistors 240, 242, 244 and an enable/disable signal. Theenable/disable signal may be generated by additional control circuitry(not shown), as desired.

The general on/off operation of the current source A 220 is controlledby transistors 224, 230, 234, and a signal TA1 output from the dataregisters 122, as shown. When the signal TA1 equals a logic one, thecurrent source A 220 is turned off and no current is output. When thesignal TA1 equals a logic zero, the current source A 220 is turned onand outputs current I_(A). Similarly, the general on/off operation ofthe current source B 222 is controlled by transistors 226, 232, 236, anda signal TA0 output from the data registers 122, as shown. When thesignal TA0 equals a logic one, the current source B 222 is turned offand no current is output. When the signal TA0 equals a logic zero, thecurrent source B 222 is turned on and outputs current I_(B). Twotransistors 246, 248 provide an enable/disable function based upon thedetection of a thermal asperity output from the comparator 108. As willbe appreciated, this function may be integrated with the output of theTA registers producing the signals TA0 and TA1.

In the preferred embodiment, the current I_(s)=0, when no thermalasperity is detected by the comparator 108, or when TA1=logic zero andTA0=logic zero. The current I_(s)=I_(B), when TA1=logic zero andTA0=logic one. The current I_(s)=I_(A), when TA1=logic one and TA0=logiczero. The current I_(s)=I_(B)+I_(A), when TA1=logic one and TA0=logicone. Also, in the preferred embodiment, the current source A 220 outputs20 microamperes and the current source B 222 outputs 10 microamperes.Therefore, the current Is is programmable to four levels: zero, 10, 20or 30 microamps.

Because the resistive value of the transistors 200, 202, 204, 206, 208,210 varies with the amount of current flowing therethrough, theresistance of the variable resistance circuit 132, and hence the cut-offfrequency (transfer function) of the high pass filter 112, varies and isprogrammable in response to the programmable current source (currentsources 220, 222). Accordingly, for different values of current I_(s),the cut-off frequency (transfer function) of the filter 112 isdifferent.

In the preferred embodiment, and under current industry specifications,the initial cutoff frequency (under normal operating conditions when nothermal asperity is detected) is around 500 KHz, and the cutofffrequency is programmable in the range between 500 KHz and 20 MHz. Asdata rates go higher, the programmable cut-off frequencies may also needto go higher. It will be understood that the factors influencing thedesired cut-off frequency (based on the value of resistance) of thefilter 112 include the programmable values of the current I_(s), thenumber, type and configuration of elements making up the variableresistance circuit 132, as well as the resistance characteristics oftransistors 200, 202, 204, 206, 208, 210.

The resistance R_(e) for a typical bipolar transistor is given by theequation R_(e)=(V_(T)/I_(E)) where V_(T) is typically 25 mV, and thisresistance is relatively linear. For example, currents of 1, 10, 100,and 1000 microamps generate resistance values of about 25 Kohms, 2.5Kohms, 250 ohms, and 25 ohms, respectively. Accordingly, a person ofordinary skill in the art can obtain desired resistance values for thevariable resistance circuit 132 and achieve the filter cut-off frequencyas desired.

Now referring back to FIGS. 4 and 6, the realization of the transitionof the cut-off frequency (FIG. 4) and the resistance of the filter (FIG.6) between time T2 and T3 will now be explained. Referring to FIG. 8,the general on/off operation of the current sources 220, 222 wasexplained above. Additional circuitry is added to achieve the slowertransition, as described earlier herein, from the programmed cut-offfrequency (transfer function) to the initial cut-off frequency (or fromthe programmed resistance to the initial resistance). The additionalcircuitry is in the form of a capacitor 236 coupled to ground and to theS/D of the transistor 232 and a capacitor coupled to ground and coupledto the S/D of the transistor 234. The capacitors 238, 236 are utilizedto slowly turn off the p-channel transistors 224, 226 coupling power tothe current sources 220, 222, respectively.

Assuming a thermal asperity is detected and the current sources 220, 222are turned on, when the thermal asperity is no longer detected, thesignals at the gates of the transistors 232, 234 are logic zero (orfloating), thereby turning off the transistors 232, 234. The transistors228, 230 then provide a current to charge the capacitors 236, 238,respectively, to a logic one, thus slowly turning off the currentsources 220, 222. Without the capacitors 236, 238, the signal at thegates of the transistors 224, 226 would rise abruptly to a logic one,thereby quickly shutting off the current sources 220, 222. Differentvalues for the capacitors 236, 238 (as well as additional circuitry, forexample, resistors, like resistors) may be used to shape the transition(change in resistance of the filter 112 and change in cut-off frequency(transfer function)) between the time T2 and T3, as desired. In thepreferred embodiment, the capacitors 236, 238 have a value of about twopicofarads. The transition as described helps eliminate or reduceunwanted offset and ringing problems.

The general operation of the present invention will now be described.Under normal operating conditions, i.e., when no thermal asperity isdetected (a thermal asperity may be present but undetected or notrelevant because of its magnitude/size), the filter 114 provides a highpass filtering function operating at a first cut-off frequency (transferfunction) for the readback signal from the read head 14. The firstcut-off frequency is determined by setting or programming the resistiveelement in the filter 114 to have a predetermined resistance value. Theamount of resistance is set or programmed in response to a first inputto a programmable current source. The first input initiates a firstpredetermined amount of current from the programmable current source andthe resistance is dependent on the magnitude of the current flowing.

Upon detection of a relevant thermal asperity upon exceeding apredetermined threshold, a second input causes the programmable currentsource to output a different, predetermined amount of current. Thischanges the value of the resistance in the filter 114 and results in adifferent, second cut-off frequency (transfer function) for the filter114. The filter 114 has been programmed to have a different transferfunction which is utilized to eliminate or reduce the effects of thethermal asperity by filtering the readback signal. In response tothermal interference detection, the filter 114 provides a high passfiltering function operating at a second first cut-off frequency(transfer function) for the readback signal from the read head 14. Inthe preferred embodiment, the second cut-off frequency is higher thanthe first cut-off frequency of the filter 114.

Different amounts of current are programmable based on different inputsto the programmable current source. Therefore, the cut-off frequency ofthe filter 114 can be set or programmed to have one of a multitude ofpossible cut-off frequencies.

When the magnitude of the relevant thermal asperity decreases and theread signal transitions back across the predetermined threshold (TAfault no longer detected), the current source is programmed to the firstamount of current thereby programming the cut-off frequency (transferfunction) of the filter 114 back to first cut-off frequency. The filter114 then again provides a high pass filtering function operating at thefirst cut-off frequency (transfer function) for the readback signal fromthe read head 14.

Another feature of the present invention operates to slowly (relativelyslowly) transition the cut-off frequency (transfer function) of thefilter 114 from the second cut-off frequency back to the first cut-offfrequency (by slowly transitioning the resistance of the variableresistance circuit 132), see FIGS. 4 and 6. Circuitry is utilized tocreate a relatively slow and predictable control signal (voltage) to theprogrammable current source to slowly change the amount of currentoutput therefrom. In the preferred embodiment, a capacitor provides thisfunction.

Although the present invention and its advantages have been described inthe foregoing detailed description and illustrated in the accompanyingdrawings, it will be understood by those skilled in the art that theinvention is not limited to the embodiment(s) disclosed but is capableof numerous rearrangements, substitutions and modifications withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A thermal event recovery circuit for filtering aread signal from a read head in a disk drive, the circuit comprising: adetection circuit for detecting a thermal event affecting the readsignal; and a filter comprising a variable resistance circuit having aresistance that varies in response to a control input, the resistanceequal to a first predetermined resistance in response to a first controlinput and equal to a second predetermined resistance in response to asecond control input, the second control input generated in response tothe detection of the thermal, event and wherein the first control inputcomprises a first current and the second control input comprises asecond current.
 2. The circuit in accordance with claim 1 wherein thefilter is a high-pass filter and the filter comprises a capacitiveelement and the variable resistance circuit comprises a fixed resistanceelement and a variable resistance element coupled in parallel with thefixed resistance element.
 3. The circuit in accordance with claim 1wherein the resistance of the variable resistance circuit equals a thirdpredetermined resistance in response to a third control input, and thesecond control input or the third control input is generated in responseto the detection of the thermal event, and wherein the third controlinput comprises a third current.
 4. The circuit in accordance with claim3 wherein the control input is in the form of a current output from acurrent source, and the first control input comprises the current havinga first magnitude, the second control input comprises the current havinga second magnitude, and the third control input comprises the currenthaving a third magnitude.
 5. The circuit in accordance with claim 1further comprising a current source outputting the first current inresponse to the first control input and outputting the second current inresponse to the second control input, wherein the first predeterminedresistance is generated in response to the first current and the secondpredetermined resistance is generated in response to the second current.6. The circuit in accordance with claim 5 wherein the resistance of thevariable resistance circuit equals a third predetermined resistance inresponse to a third control input, the current source outputs a thirdcurrent in response to the third control input and the thirdpredetermined resistance is generated in response to the third current,and the second control input or the third control input is generated inresponse to the detection of the thermal event.
 7. The circuit inaccordance with claim 1 wherein the first and second predeterminedresistances are controlled using a current.
 8. The circuit in accordancewith claim 1 wherein the resistance of the variable resistance circuitslowly transitions during a transition period from the secondpredetermined resistance to the first predetermined resistance after thethermal event is no longer detected.
 9. The circuit in accordance withclaim 8 wherein the transition period is greater than about 50nanoseconds.
 10. The circuit in accordance with claim 8 wherein thetransition follows a transition curve and the transition period isgreater than about 50 nanoseconds.
 11. The circuit in accordance withclaim 10 wherein the transition curve is nonlinear.
 12. The circuit inaccordance with claim 1 further comprising transition circuitry fortransitioning during a transition period the resistance of the variableresistance circuit from the first predetermined resistance to the secondpredetermined resistance after the thermal event is no longer detected.13. A thermal event recovery circuit for filtering a read signal from aread head in a disk drive, the circuit comprising: a detection circuitfor detecting a thermal event affecting the read signal; and a filterhaving a first predetermined transfer function in response to a firstinput, and having a second predetermined transfer function in responseto a second input, the second input generated in response to thedetection of the thermal event, and wherein the first input comprises afirst current and the second input comprises a second current.
 14. Thecircuit in accordance with claim 13 wherein the filter is a high-passfilter and the filter comprises a capacitive element and a resistiveelement, the resistive element comprising a fixed resistance element anda variable resistance circuit coupled in parallel with the fixedresistance element.
 15. The circuit in accordance with claim 13 whereinthe filter has a third predetermined transfer function in response to athird input, and the second input or the third input is generated inresponse to the detection of the thermal event, and wherein the thirdinput comprises a third current.
 16. The circuit in accordance withclaim 13 wherein the transfer function of the filter slowly transitionsduring a transition period from the second transfer function to thefirst transfer function after the thermal event is no longer detected.17. The circuit in accordance with claim 16 wherein the transitionperiod is greater than about 50 nanoseconds.
 18. A circuit for detectingthermal interference and filtering a read signal from a read head in adisk drive, the circuit comprising: a detection circuit for detectingthermal interference affecting the read signal; a current source foroutputting a first current or second current; and a filter comprising, afixed resistive element generating a first resistance, and a variableresistance circuit generating a second resistance that varies inresponse to the output of the current source, the second resistanceequal to a first predetermined resistance in response to the firstcurrent and equal to a second predetermined resistance in response tothe second current, the second current generated in response to thedetection of the thermal interference.
 19. The circuit in accordancewith claim 18 wherein the current source outputs a third current and thesecond resistance equals a third predetermined resistance in response tothe third current, the second current or the third current generated inresponse to the detection of the thermal event.
 20. The circuit inaccordance with claim 18 wherein the second resistance of the variableresistance circuit slowly transitions during a transition period fromthe second predetermined resistance to the first predeterminedresistance after the thermal event is no longer detected.
 21. Thecircuit in accordance with claim 20 wherein the transition period isgreater than about 50 nanoseconds.
 22. A disk drive comprising: astorage medium, a read head for detecting data on the storage medium andgenerating a read signal; a detection circuit for detecting a thermalevent affecting the read signal; and a filter comprising a variableresistance circuit having a resistance that varies in response to acontrol input, the resistance equal to a first predetermined resistancein response to a first control input and equal to a second predeterminedresistance in response to a second control input, the second controlinput generated in response to the detection of the thermal event, andwherein the first control input comprises a first current and the secondcontrol input comprises a second current.
 23. The circuit in accordancewith claim 22 wherein the filter is a high-pass filter and the filtercomprises a capacitive element and the variable resistance circuitcomprises a fixed resistance element and a variable resistance elementcoupled in parallel with the fixed resistance element.
 24. The circuitin accordance with claim 22 wherein the resistance of the variableresistance circuit equals a third predetermined resistance in responseto a third control input, and the second control input or the thirdcontrol input is generated in response to the detection of the thermalevent, and wherein the third control input comprises a third current.25. The circuit in accordance with claim 22 further comprising a currentsource outputting the first current in response to the first controlinput and outputting the second current in response to the secondcontrol input, wherein the first predetermined resistance is generatedin response to the first current and the second predetermined resistanceis generated in response to the second current.
 26. The circuit inaccordance with claim 22 wherein the resistance of the variableresistance circuit slowly transitions during a transition period fromthe second predetermined resistance to the first predeterminedresistance after the thermal event is no longer detected.
 27. A methodof reducing the affects of a thermal asperity on a read signal in a diskdrive, comprising the steps of: filtering the read signal with a filterhaving a first transfer function; detecting thermal interference in theread signal; changing, in response to the detection of the thermalinterference, the first transfer function of the filter to a secondtransfer function, the first transfer function dependent upon a firstpredetermined resistance of the filter and the second transfer functiondependent upon a second predetermined resistance of the filter, andwhereby the first predetermined resistance is generated in response to afirst current and the second predetermined resistance is generated inresponse to a second current; filtering the signal with the filterhaving the second transfer function; and transitioning, after apredetermined time period has elapsed after the detection of the thermalinterference, the second transfer function of the filter to the firsttransfer function.
 28. The method in accordance with claim 27 whereinthe first transfer function comprises substantially a high pass transferfunction having a first cut-off frequency and the second transferfunction comprises substantially a high pass transfer function having asecond cut-off frequency.
 29. The method in accordance with claim 27further comprising the step of selecting one of a plurality of transferfunctions as the second transfer function.
 30. The method in accordancewith claim 27 wherein the step of transitioning the second transferfunction of the filter to the first transfer function occurs over atransition period greater than about 50 nanoseconds.
 31. A thermal eventrecovery circuit for filtering a read signal from a read head in a diskdrive, the circuit comprising: a detection circuit for detecting athermal event affecting the read signal; and a filter comprising avariable resistance circuit having a resistance that varies in responseto a control input, the resistance equal to a first predeterminedresistance in response to a first control input and equal to a secondpredetermined resistance in response to a second control input, thesecond control input generated in response to the detection of thethermal event, and wherein the resistance of the variable resistancecircuit equals a third predetermined resistance in response to a thirdcontrol input, and the second control input or the third control inputis generated in response to the detection of the thermal event.
 32. Athermal event recovery circuit for filtering a read signal from a readhead in a disk drive, the circuit comprising: a detection circuit fordetecting a thermal event affecting the read signal; and a filtercomprising a variable resistance circuit having a resistance that variesin response to a control input, the resistance equal to a firstpredetermined resistance in response to a first control input and equalto a second predetermined resistance in response to a second controlinput, the second control input generated in response to the detectionof the thermal event, wherein the resistance of the variable resistancecircuit slowly transitions during a transition period greater than about50 nanoseconds from the second predetermined resistance to the firstpredetermined resistance after the thermal event is no longer detected.33. A thermal event recovery circuit for filtering a read signal from aread head in a disk drive, the circuit comprising: a detection circuitfor detecting a thermal event affecting the read signal; and a filterhaving a first predetermined transfer function in response to a firstinput, and having a second predetermined transfer function in responseto a second input, the second input generated in response to thedetection of the thermal event, and wherein the filter has a thirdpredetermined transfer function in response to a third input, and thesecond input or the third input is generated in response to thedetection of the thermal event.
 34. A thermal event recovery circuit forfiltering a read signal from a read head in a disk drive, the circuitcomprising: a detection circuit for detecting a thermal event affectingthe read signal; and a filter having a first predetermined transferfunction in response to a first input, and having a second predeterminedtransfer function in response to a second input, the second inputgenerated in response to the detection of the thermal event, and whereinthe transfer function of the filter slowly transitions during atransition period greater than about 50 nanoseconds from the secondtransfer function to the first transfer function after the thermal eventis no longer detected.
 35. A disk drive comprising: a storage medium, aread head for detecting data on the storage medium and generating a readsignal; a detection circuit for detecting a thermal event affecting theread signal; and a filter comprising a variable resistance circuithaving a resistance that varies in response to a control input, theresistance equal to a first predetermined resistance in response to afirst control input and equal to a second predetermined resistance inresponse to a second control input, the second control input generatedin response to the detection of the thermal event, and wherein theresistance of the variable resistance circuit equals a thirdpredetermined resistance in response to a third control input and thesecond control input or the third control input is generated in responseto the detection of the thermal event.
 36. A thermal event recoverycircuit for filtering a read signal from a read head in a disk drive,the circuit comprising: a detection circuit for detecting a thermalevent affecting the read signal; and a filter comprising a variableresistance circuit having a resistance that varies in response to acontrol signal having a first current state and a second current state,the resistance equal to a first predetermined resistance in response tothe first current state and equal to a second predetermined resistancein response to the second current state, the first current statecomprising a current having a first magnitude and the second currentstate comprising a current having a second magnitude.
 37. A thermalevent recovery circuit for filtering a read signal from a read head in adisk drive, the circuit comprising: a detection circuit for detecting athermal event affecting the read signal; and a filter having a firstpredetermined transfer function in response to a control signal having afirst current state, and having a second predetermined transfer fucntionin response to the control signal having a second current state, thefirst current state comprising a current having a first magnitude andthe second current state comprising a current having a second magnitude.38. A thermal event recovery circuit for filtering a read signal from aread head in a disk drive, the circuit comprising: a detection circuitfor detecting a thermal event affecting the read signal; and a filtercomprising a variable resistance circuit having a resistance that variesin response to a control input having a first state and a second state,the resistance equal to a first predetermined resistance in response tothe first state and equal to a second predetermined resistance inresponse to the second state, the second state generated in response tothe detection of the thermal event, wherein the resistance of thevariable resistance circuit slowly transitions during a transitionperiod greater than about 50 nanoseconds from the second predeterminedresistance to the first predetermined resistance after the thermal eventis no longer detected.
 39. A thermal event recovery circuit forfiltering a read signal from a read head in a disk drive, the circuitcomprising: a detection circuit for detecting a thermal event affectingthe read signal; and a filter having a first predetermined transferfunction in response a control input having a first state, and having asecond predetermined transfer function in response to the control inputhaving a second state, the second state generated in response to thedetection of the thermal event, and wherein the transfer function of thefilter slowly transitions during a transition period greater than about50 nanoseconds from the second transfer function to the first transferfunction after the thermal event is no longer detected.